High vacuum fuse



7, 1933- D. c. PRINCE ET AL 1,934,459

HIGH VACUUM FUSE Original Filed March 23, 1928 Fig. I

Inventobs: David C. Phince. Behbham \A ellman.

Theih AttOYnQH.

Patented Nov. 7, 1933 HIGH VACUUM FUSE David 0. Prince and Bertram Wellman, Schenectady, N. Y., aasignors to General Electric Company, a corporation of New York Original application March 23;, 1928, Serial No.

Divided and this application July 22,

1925. 1. Serial No. 379,976

4 Claims.

Gill invention relates to fuses for interrupting electric circuits, and particularly to enclosed fuses of the direct-current type, comprising a combination of elements which cooperate in a way to 5 secure new results by utilizing certain principles of electron emission in a very high vacuum.

A principal object of our invention is to provide a high-tension, high-vacuum fuse of the direct-current type which shall interrupt currents at high voltages without prolonged arcing and within a brief interval of time.

A further object is to provide a fuse which when it has blown leaves a, gap of exceptionally high dielectric strength between the terminals of the fuse whereby there is no danger from subsequent breakdown across the gap.

A further object is the provision of fuse terminals composed of a. refractory metal, which are so constructed that maintenance thereof at a temperature permitting thermionic emission of electrons, as when the fusible element is ruptured by predetermined flow of current, is precluded.

Among the advantages of our invention whic are of great commercial importance are that the fuse reduces fire hazard to a minimum; is noiseless in operation; relatively small, relatively inexpensive, and quite definite in its action of interruptlng the circuit when that-current value for which the fuse is designed to operate is reached.

In accordance with our invention the fuse is maintained in a very high vacuum which not only protects the fuse metal from oxidation or other deteriorating influences but presents such deflnite conditions for loss of heat by conduction and radiation that the fuse operates with considerably greater uniformity at its designed current value.

Further in accordance with our invention, the fuse when connected in a. direct-current circuit, functions to interrupt the current when the fuse blows by reason of rapid cooling of the cathode and consequent cessation of electron flow therefrom.

This application is a division of our cc-pending application, Serial No. 264,113. filed March 23, 1928, for "High vacuum fuses.

Our invention will be more fully set forth in the following description referring to the accompanying drawing, and the features of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the drawing, 1 represents a. fuse em" bodying our invention; Fig. 2 represents a modifled form of fuse vessel; and Fig. 3 represents a modified construction of fuse embodying broad features of our invention together with certain imp1oveznents.

In Fig. l we have represented one example of a fuse embodying our invention. In this embodlmerit the vessel is indicated as made of vitreous material such as quartz or a suitable glass, ex-

amples of 1 which are well known in the art of electron discharge devices, X-ray apparatus, and the like. As here shown the vessel is provided with a bulbous central portion 1. The fuse link 2 is located centrally of the bulbous portion and the ends of the fuse link are secured in any suit- 7Q able manner to terminals 3 and 4 which are connected by the conductors 5 and 6 to tenninal caps 7 and 8 respectively, which are indicated as extending over and secured in any suitable manner as, for example, by cement, to the neck pcrw tions of the vessel. The cylindrical portions of the-caps 7 and 8 are adapted to be inserted in fuse-holding members or clips not illustrated.

Theterrninal members 3 and 4 of the directcurrent fuse must be composed of a refractory metal having a high melting point, such as tungsten, for example, in order that there will be no cathode-spot efiect, as hereinafter described. As

is well known, a refractory metal, such as tungsten, is adapted when heated sufficiently to emit electrons thermionically, as contrasted with fuel:- als suchas copper, silver, and nickel, which are of such low melting points that thermionic emission of electrons therefrom does not occur. The material of which the fuse link itself is made is on not of the same importance, but tungsten is known tobe a satisfactory metal and has the advantage that it can be raised to a very high temperature by passing the heating current therethrough while the vessel is being evacuated so that the fuse link is quite completely freed of occluded gases.

Where the fuse is to carry moderatevalues of current the leading-in conductors 5 and 6 may be sealed to the stems 9 and 10 of the vessel by a pinch seal. Examples of suitable leading-m conductors adapted to make a tight and permanent seal with glass are now well known in the art of incandescent lamps and hlgh-vacuum devices. Such leading-in wires are described for example in Letters Patent to Fink No. messes, dated June 24, 1924. The ends of the stems g and 10 are provided with shields ll. and 12 to protect the seals against the injurious effect of electron bombardment during the anglerstionv of the fuse in interrupting a circuit as hereinafter described.

The shields 11 and 12, when disposed adjacent the terminals 3 and 4 of the fuse may likewise serve to aid in the dissipation of heat from the terminals by reason of the increased heat radiating surfaces of the shields. The shields may be of metal such as molybdenum since this metal has the advantage that when used in highvoltage, high-vacuum electron discharges it may be readily raised to a suiliciently high temperature by high frequency heating to drive out the occluded gases therefrom during exhaust.

The very best technique for securing an extremely high vacuum is important for securing the best results when the fuse operates. The vessel should be baked during exhaust to drive out occluded gases from the walls and all surfaces and metal parts within the vessel should be freed from occluded gases. It is preferable to degas metal parts such as the terminal members 3 and 4 and also the shields 11 and 12 before they are assembled in the vessel. As pointed out in Letters Patent to Langmuir No. 1,558,436, dated October 20, 1925, metal which has been freed from occluded gas does not readily reabsorb gas even though it is exposed to the air or other gases. This makes it possible thoroughly to de-gas such parts as desired before assembling in the vessel and facilitates the final operation of de-gasing and exhausting after assembly.

After the fuse elements have been assembled in the vessel sufficient current should be passed through the fuse to raise it to a temperature sufficient to drive out occluded gases while the exhausting operation is going on. The walls of the vessel should be thoroughly baked out and suitable means utilized, for example high-frequency heating, to assist in raising the metal parts in the vessel to a sufficiently high temperature to in sure that after the vessel has been sealed off an extremely high vacuum will continue to be maintained although the fuse may become very hot when carrying currents approximating the value at which the fuse is designed to interrupt the circuit.

In the drawing, the projection 13 indicates one suitable place at which the vessel may be sealed off after the desired high vacuum has been produced. As indicated in the drawing, this location of the projection enables it to be covered by the metal fuse caps and protected from injury during handling of the fuse.

Experience with fuses of the high-vacuum type described will enable one to determine from the appearance of a fuse when a predetermined testing current is passed through it whether or not the vacuum is of the high order desired before the fuse is put into service on a line. If desired, however, any well known method of testing such a fuse may be employed to determine whether or not the vacuum is of the desired order.

In order better to understand the new functions and results of our invention, we will now describe the action which we believe explains why the fuse will interrupt a high-voltage circuit within a very brief interval of time and without the usual arcing or explosive defect attendant upon high-tension fuses of previous construction. At this point it should be borne in mind that there are two types of electron emission. Thermlonic emission is produced by a heating of a metal to the point where the electrons in it have sufficient energy to break through the metal boundary into space. This emission phenomenon is described by the Richardson Law, and is an important factor in the operation of our directcurrent fuse. The second kind of electron emission we will call cathode-spot emission." The theory which we believe to explain the lastnamed type of emission is set forth in detail in our co-pending application above referred to. and the following outline of the theory is believed to suffice for an understanding of the present invention.

In the event that terminals 3 and 4 are composed of a metal having a low melting point, such as copper, silver, or nickel, for example, rupture of the fusible element produces a very high po tential gradient at that point of the cathode to which the fuse link was attached, whereby electrons are pulled out of the soft metal terminal at a temperature much lower than that corresponding to emission from a hot cathode of refractory metal such as tungsten. These electrons in turn collide with the vaporized metal in the space formerly occupied by the fusible element to ionize some of the metal vapor, thus producing additional electrons, the positive ions traveling to the cathode to accumulate there and maintain the high potential gradient necessary for continuing the flow of electrons, and also serving to maintain the cathode spot at high temperature by their constant bombardment. Accordingly, it is seen that the flow of electrons from the cathode, once it is started, continues as long as the conditions originally present obtain.

The flow of electrons by thermionic emission, however, necessitates that the cathode be heated to such temperature that electrons are emitted therefrom without regard to any cathode-spot effect. As previously stated, a refractory metal such as tungsten may be heated to such a temperature that thermionic emission therefrom occurs. However, when the temperature of the metal decreases to below this point electron emission practically immediately ceases. In other words, in order for thermionic emission from a metal such as tungsten to be started and maintained, the metal n'iust not only be raised to the 126 high temperature necessary to start such emission, but must be continuously maintained so. With the terminals 3 and 4 composed of a refractory metal, as tungsten, rupture of the fusible element 2, due to excessive current traversing the same, may momentarily cause the cathode to be heated sufficiently to emit electrons thermionically so that current will continue to flow but will tend to reduce in value as the gap across which the discharge is taking place increases. The terminals, and particularly the cathode, should be so constructed that the heat due to rupture of the fusible element may be rapidly conducted away and dissipated, and they should therefore preferably be ofreasonably large size unless special cooling means are provided.

It is difficult to secure sufficient metal vapor from the tungsten cathode to maintain a cathode spot. Several hundred amperes at 250 volts have been tried and found insufficient where tungsten 1 0 contacts are separated-in a high vacuum. With direct current flowing, the cathode will continuously be cooled by the emission of electrons and by conduction through and radiation from the metal until the temperature is too low to emit further electrons thermionically after which the circuit will be opened. The small amount of vapor produced from the tungsten cathode is not sufficient to result in a positive ion bombardment of the cathode sufficient to restore as much heat figure the fuse-supporting terminals 22 and 23 are made in the form of hairpinlike loops. For directcurrent service these loops are made of a refractory metal having a high melting point, such as tungsten for example, and the final operation of depriving the loop terminals of occluded gas is facilitated by the fact that a local heating current may be passed through each of the loops by connecting the leading-in wires for the respective loops to suitable sources of current. Cup rents can thus be utilized which are as large as necessary without dependence upon the amount of current the fuse link itself can carry. Loops of considerable size can thereby be readily brought to the temperature necessary to free them from occluded gases while the final exhaust ing operation is taking place and the necessary current can also be passed from a suitable source of current through the fuse link so as to bring it also to the desired high temperature. The arrangement shown in Fig. 3 is disclosed and claimed in the co-pending application of David C. Prince, Serial No. 264,117, filed concurrently herewith and assigned to the same asslgnee as the present application.

While we have illustrated certain embodiments of our invention and described its theory of operation as we understand it, and described certain relations of utility for our invention, it is to be understood that we are not bound by any theory of operation, and it is moreover apparent that modifications and changes will occur to those skilled in the art without departing from our invention, and it is contemplated that our fuse may find a greater field or use in some application other than we have described and we therefore aim in the appended claims to cover all modifications and applications within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. A high vacuum direct current fuse comprising an evacuated vessel, terminals located within said vessel, and a fusible element connected to and interposed between said terminals, all of the elements within the vessel as well as the walls of the vessel being thoroughly freed of occluded gas, said terminals being relatively large with respect to the fusible element and at least one of the said terminals being composed of a refractory metal whereby said terminal tends to cool when operating as a cathode and thereby to reduce electron emission therefrom and stop the flow of current.

2. A direct-current fuse of the high vacuum type comprising a sealed vessel having a vacuum of the order of about one-thousandth of a micron, spaced terminal members within said vessel composed of a metal having a high melting point, said metal being capable of thermionic electron emission, and a fusible element interconnecting said terminal members.

3. A direct-current fuse of the high vacuum type comprising a sealed vessel having a vacuum of they order of about one-thousandth of a micron, spaced terminal members composed of tungsten disposed within said vessel, and a fusible element interconnecting said terminal members, said terminal members having comparatively large heat-dissipating surfaces with reference to the current traversing the terminals during rupture of the fusible element for rapidly cooling said members.

4. A direct-current fuse of the high vacuum type comprising a sealed vessel having a vacuum lift approximately of the order of from .01 to .001: micron, spaced terminal members composed of l'lll 

